As a method for measuring the concentration of a particular component in a sample such as blood, an electrochemical method is known which utilizes a biosensor provided with a reagent layer and a pair of electrodes. In this method, the sample is introduced to the reagent layer to cause the particular component in the sample to react with a component in the reagent layer. On the other hand, a voltage is applied to the reagent layer by utilizing the electrodes, and the current between the electrodes is measured. The current changes in accordance of the degree of the reaction. Further, the degree of reaction depends on the concentration of the particular component in the sample. The concentration of the particular component in the sample can be computed based on the measurements of the current.
In such analysis, it is desirable that whether or not conditions necessary for performing the analysis are satisfied can be checked in advance. For instance, when the amount of the sample introduced to the reagent layer is insufficient, accurate analysis results cannot be obtained. Further, accurate analysis results may not be obtained due to a defect of the biosensor. When the analysis is carried out without noticing such a fail condition, the user may believe such inaccurate analysis results to be accurate.
JP-B-2800981 discloses a method for detecting such a fail condition as described above. In this method, fail detection is performed by applying a voltage across a pair of electrodes of a biosensor and measuring the current between the electrodes. FIG. 8 shows the change of the current in this method. As shown in the figure, the current between the electrodes continues to increase after a time point indicated by the reference sign n1 at which a sample is introduced to the reagent layer. When the current value exceeds a predetermined threshold value at the time point ta, the introduction of the sample is detected. The rate at which the current increases immediately after the sample introduction depends on the amount of the sample introduced to the reagent layer. The smaller the amount of introduced sample is, the lower the rate at which the current increases. Subsequently, the current at a time point tb after a predetermined period T has elapsed since the time point ta is measured. When the measured current is higher than a predetermined second threshold value as shown in the figure, it is determined that the amount of introduced sample is sufficient. On the other hand, when the measured current is no higher than the second threshold value, it is determined that the amount of introduced sample is insufficient.
In this way, in the prior art method, fail determination is made based on the difference between two current values at predetermined time points. In other words, the determination is made based on the slope of the curve representing the current change at a predetermined time point, i.e., the rate of the current change.
However, the prior art method has following problems.
Although the rate of the change of the current flowing between the electrodes varies depending on the amount of introduced sample as noted above, the variation of the rate of the current change is not sufficiently large when the change of the amount of the sample introduced to the reagent layer is small. Therefore, in the prior art method, it is sometimes difficult to accurately determine whether or not the amount of introduced sample is sufficient. In the prior art method, to eliminate the possibility that the amount of introduced sample is erroneously determined to be sufficient although it is actually insufficient, an excessively large value need be set as the second threshold value Th2. In such a case, however, there is a possibility that the amount of introduced sample is often determined to be insufficient although it is actually sufficient.